Large variation in participant eligibility criteria used in plantar heel pain research studies - a systematic review

Introduction
Variable eligibility criteria across studies on plantar heel pain may result in compromising the generalisability of meta-analyses when heterogeneity is not accounted for. We aimed to explore: (i) heterogeneity of participant eligibility criteria in studies that have investigated plantar heel pain, and (ii) associations between key eligibility criteria and the characteristics of the participants included in the study.

Methods
In this systematic review with narrative synthesis, we extracted participant eligibility criteria, and participants’ age, body mass index (BMI), symptom duration and pain level from published studies on plantar heel pain. We performed a content analysis of criteria and aligned overarching criteria to the International Classification of Functioning, Disability and Health (ICF). We pooled studies that used the same thresholds for participant eligibility criteria into sub-groups. We also pooled and reported studies that did not have any eligibility criteria for the quantitative characteristics to use their data for reference values and pooled studies that did not have any eligibility criteria for the characteristics as reference.

Results
Two hundred and fourteen articles were included. The most reported participant eligibility criteria (as aligned to the ICF) related to body structures/function and personal factors. Age, BMI, symptom duration and pain level were used with various ranges and/or thresholds across studies (age was reported in 23 different ways across 97 studies; BMI 7/13; symptom duration 14/100; and pain level 8/31). When eligibility criteria included thresholds close to the reference value of a participant characteristic, characteristics were associated with criteria (e.g., younger participants when an upper age threshold was used).

Conclusion
Participant eligibility criteria in studies on plantar heel pain vary widely; studies differed substantially in their use of quantitative thresholds. Participant characteristics of samples in studies were associated with the criteria used. This study emphasises a need for adjusting for participant heterogeneity in systematic reviews to improve their validity.